The present invention relates to a semiconductor device such as a data processor, a microprocessor or a microcomputer, which includes a CPU and a signal-processing circuit. More particularly, the present invention relates to an effective technique adapted to generation of a side tone in a hand telephone (a cellular phone) or the like
Apparatuses such as a hand phone for transmitting and receiving an audio signal in duplex communications have a side-tone function. The side-tone function is a function to combine a voice of the user input from a microphone with a voice received from a communication partner and output the combined voices to a speaker during a conversational communication. For example, in Japanese Patent Laid-open No. Hei 7-240782, there is disclosed a hardware technology whereby an audio signal to be transmitted from an A/D converter is supplied to a variable-gain amplifier, being converted into a side tone added to a voice received from a communication partner during a conversational communication before being output to a speaker through D/A conversion. In addition, a processor is also capable of carrying out side-tone processing by execution of software as is disclosed in Japanese Patent Laid-open No. Hei 9-284364.
With the hardware technology for generating a side tone as described above, however, external circuit components such as an amplifier to be added externally to a processor are required, making it virtually impossible to apply the technology to fields like a hand phone where requirements such as a low cost, a small size and thinness are demanded. In the case of the software for side-tone processing, on the other hand, the inventors of the present invention discovered a problem that the processor or a CPU embedded in the processor is occupied by side-tone processing for each audio data so that it is feared that other software processing becomes stagnant. Examples of the other software processing include encoding and decoding processes, compression and error correction based on a control by the CPU.